Colorectal carcinoma is one of the most prevalent cancers in the United States and one of the leading causes of cancer mortality. While diet, family history, and lifestyle factors have been proposed as risk factors for colorectal cancer, the etiology of the majority of colorectal cancers remains uncertain. Studies evaluating medication use and colorectal cancer risk have mainly focused on only a few drug classes. Experimental and observational studies support a potential role for a number of medications. The goal of this study is to evaluate the relationship between understudied but commonly prescribed medication use and colorectal cancer risk. To meet these aims, we plan to conduct a case-control study of colorectal cancer comprised of 675 cases and 675 controls in the defined population of Group Health Cooperative, a large health maintenance organization in western Washington State. Cancer outcomes will be identified through the western Washington Surveillance, Epidemiology, and End Results registry. Data on medication use, medical conditions, and covariates will be obtained from traditional medical records and automated data files, which include pharmacy records, laboratory values, hospitalizations, outpatient visits, and cause of death.